Currently, the 3 accounts for USD, EUR and GBP are all held with Barclays in London using UK sortcode / bankaccount number / IBAN.
For GBP this is the perfect “native” solution, for EUR this is also OK as funds can be moved to/from this account without incurring costs (possible caveat Brexit).
For USD this is already problematic: Transferring USD to an account in the UK will, in most cases, attract fees. Once new currencies are introduced, the same problem will apply. Example CHF. If the CHF-account will be a Barclays-CHF account in the UK, loading the card in CHF will attract substantial fees.
Possible solution: See currencyfair / transferwise. These companies hold local accounts in the respective countries, allowing for free transfers.